Optimizing defragmentation operations in a differential snapshotter

ABSTRACT

A differential snapshot is established and maintained for a set of files stored on a volume. Copy-on-write operations are avoided for logically insignificant moves of blocks, such as the block rearrangements characteristic of defragmentation utilities. A file system passes a block copy command to lower-level drivers that are to inform the snapshotter that a block move operation is not logically meaningful. When the logically insignificant move is of a block whose data forms part of the data captured in the snapshot virtual volume, and when the move is to a block location that is functioning as logical free space, the snapshotter can simply modify its block bitmap and update translation table entries without needing to perform a copy-on-write.

CROSS-REFERENCE TO RELATED APPLICATIONS

This invention claims priority based on U.S. Provisional PatentApplication Ser. No. 60/419,252, filed on Oct. 16, 2002, which is herebyincorporated in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates generally to data storage, and moreparticularly to snapshots of file system volumes.

BACKGROUND OF THE INVENTION

Data storage is an essential feature of computer systems. Such storagetypically includes persistent data stored on block-addressable magneticdisks and other secondary storage media. Persistent data storage existsat several levels of abstraction, ranging from higher levels that arecloser to the logical view of data seen by users running applicationprograms, to lower levels that are closer to the underlying hardwarethat physically implements the storage. At a higher, logical level, datais most commonly stored as files residing in volumes or partitions,which are associated with one or more hard disks. The file system, whichcan be regarded as a component of the operating system executing on thecomputer, provides the interface between application programs andnonvolatile storage media, mapping the logically meaningful collectionof data blocks in a file to their corresponding physical allocationunits, or extents, located on a storage medium, such as clusters orsectors on a magnetic disk.

Users and administrators of computer systems benefit from having theability to recover earlier versions of files stored on the system. Usersmay accidentally delete or erroneously modify files. An administrator ofa system that has become corrupted may wish to recover the entire stateof a file system at some known good time before the corruption occurred.The underlying disk hardware can fail. A snapshot is one technique forfacilitating the recovery of earlier versions of files.

A snapshot of a volume is a virtual volume representing a point in timeon the original volume. Some snapshotters capture the point-in-time databy mirroring the entire contents of the volume in its snapshot state. Bycontrast, differential snapshotters do not make actual copies at thetime of the snapshot. Rather, changes to the original volume arecarefully monitored so that the virtual volume (i.e., the snapshot) canalways be produced. A differential snapshotter will copy a block in thevolume only if it is modified after the snapshot is taken; such a copyoperation is called a “copy-on-write.” The snapshot state of the volumecan be reconstructed by using these copies of changed blocks along withthe unchanged blocks in the original volume. In the usual case, manyfiles in the volume will be left unchanged following the snapshot, sodifferential snapshotters provide a more economical design thannondifferential approaches. As many changes occur to the originalvolume, however, a differential snapshotter must keep a large area ofdisk space to hold the older versions of the disk blocks being changed.

In most operating systems, the extents that make up the physicalallocation units implementing a particular file may be discontiguous, asmay the pool of allocation units available as logically free space foruse in future file space allocation. A disk volume in such a state issaid to be externally fragmented. In many such operating systems, avolume can be expected to suffer from increasing external fragmentationover time as files are added, deleted and modified. Externalfragmentation increases the time necessary to read and write data infiles, because the read/write heads of the hard disk drive will have toincrease their lateral movement to locate information that has becomespread over many non-contiguous sectors. If fragmentation issufficiently severe, it can lead to significantly degraded performanceand response time in the operation of the computer system.

Defragmentation utility programs provide an important remedy for datastorage systems that are prone to external fragmentation. Theseutilities can be periodically run to rearrange the physical location ofa volume's file extents so that contiguity of allocation blocks isincreased and disk read/write access time is correspondingly reduced,improving performance. A defragmentation operation consists of movingsome blocks in a file to a location that is free on the volume. Moreprecisely, the contents of one block are copied to the free blocklocation. The old location of the block becomes free and the newlocation of the block becomes occupied space. The defragmentation of avolume will typically involve an extensive number of such block moves.

Although users of file systems benefit from the disk speed optimizationsachieved by defragmentation, the benefit has come at the expense ofefficient use of differential snapshotters. If a volume is defragmentedsubsequent to the taking of a snapshot, the snapshotter will ensure thateach data block relocation by the defragmenter is preceded by acopy-on-write of the block. The logical view of the original volume isunchanged by the defragmentation operations, but because the disk blockson which the disk is physically manifested change drastically incontent, the amount of space needed to maintain the snapshot explodes.This disk space explosion may be enough to destroy a principal reasonfor using differential snapshotters in the first place, that of diskspace economy.

The problem seen in the interaction between differential snapshottersand defragmentation operations is that, prior to the present invention,differential snapshotters have not been able to distinguish logicallysignificant writes of blocks from logically insignificant block moves,treating both as requiring copy-on-write protection. This problem isparticularly acute when there is a volume defragmentation operation onthe original volume, but those of skill in the art will appreciate thatother file-manipulating programs besides defragmenters may require thenonlogical relocation or shuffling of file blocks. For example, aprogram might, for performance reasons, create a file of a particularsize and arrange the blocks in a desired way before proceeding withfurther use of the file for writing data. Prior to the presentinvention, differential snapshotters have treated such blockrearrangements as requiring copy-on-write protection.

It can be seen, then, that there is a need for an improvement indifferential snapshotters so that logically insignificant moves ofblocks from one volume location to another are recognized as notrequiring copy-on-write protection in principle. The availability ofmore efficient differential snapshotters will make more likely the useof snapshots applied on a longer-term basis for data recovery. Moreover,such an improvement will lead to greater use of defragmentationutilities and therefore will allow disk speed optimizations to takeplace while having snapshots with little performance impact and littledisk space consumed.

SUMMARY OF THE INVENTION

The present invention provides a method for capturing and maintaining adifferential snapshot of an original volume in which logicallysignificant modifications of blocks, which require copy-on-writeprotection, are distinguished from logically insignificant block moves,which in principle do not need to be preceded by copy-on-writeoperations. The invention involves the use of a file system with theability to pass a BLOCK_COPY command down to lower-level, block-orienteddrivers, a capacity not available in previous file systems, whichenables such drivers to take advantage of hardware acceleration for datablock movements. In particular, a snapshot driver, informed by the filesystem that a requested operation is a nonlogical block move, uses thisenrichment in knowledge to avoid unnecessary copy-on-write operations.Instead, the snapshotter simply updates the translation table datastructures it employs to keep track of which blocks must be protected bycopy-on-write operations and where the snapshot versions of blocks arebeing stored.

Those skilled in the art will readily perceive that the presentinvention is also applicable to differential snapshots of files andvolumes contained on block devices other than magnetic disk media and tothe use of differential snapshotters to reconstruct time-definedversions of other persistent data structures. Other aspects andadvantages of the invention will become apparent from the followingdetailed description, taken in conjunction with the accompanyingdrawings, illustrating by way of example the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating the steps taken in an embodimentof the invention with respect to a block move from a block location A toa block location B;

FIG. 2 is a flow diagram illustrating the steps taken under twoscenarios in an embodiment of the invention following the block movefrom A to B of FIG. 1 in the case where, before the move, thesnapshotter bitmap bit for block B is set and the bitmap bit for block Ais clear, with FIG. 2A illustrating the scenario where A is written, andwith FIG. 2B illustrating the scenario where there is a write to B;

FIG. 3 is a flow diagram illustrating the steps taken in an embodimentof the invention following the block move from A to B of FIG. 1 in thecase where, before the move, the bitmap bit for block B is set and thebitmap bit for block A is clear, and where, after the move, a write of Bhas not yet occurred and a move of block B to a block location C isinitiated;

FIG. 4 illustrates one possible computer in the context of which anembodiment of the present invention may be practiced;

FIG. 5 illustrates an exemplary multi-level secondary storage systemassociated with a computer, such as the computer of FIG. 4, in thecontext of which an embodiment of the present invention may bepracticed;

FIG. 6 is a diagram presenting a detailed example of the handling of alogically significant block write in an embodiment of the invention,with FIG. 6A providing the view before the write and FIG. 6B providingthe view after the write;

FIG. 7 is a diagram presenting a detailed example of the handling of asimple logically insignificant block move in an embodiment of theinvention, with FIG. 7A providing the view before the block move andFIG. 7B providing the view after the move;

FIG. 8 is a diagram continuing the detailed example of FIG. 7,presenting the handling of two logically significant block writerequests in an embodiment of the invention, including a write at a blocklocation from which a data block was nonlogically moved, and a write atthe block location to which that block was moved, with FIG. 8A providingthe view before the writes and FIG. 8B providing the view after thewrites;

FIG. 9 is a diagram continuing the detailed example of FIG. 7,presenting the handling of a second logically insignificant block movefollowing the first move depicted in FIG. 7, with FIG. 9A providing theview before the block move and FIG. 9B providing the view after themove;

FIG. 10 is a diagram continuing the detailed example of FIG. 9,presenting the handling of a third logically insignificant block movefollowing the second move depicted in FIG. 9, where the move is to theoriginal block location as presented in FIG. 7, with FIG. 10A providingthe view before the block move and FIG. 10B providing the view after themove;

FIG. 11 is a flow diagram presenting a high-level view of the stepstaken in an embodiment of the invention with respect to capturing andmaintaining the snapshot;

FIG. 12 is a flow diagram presenting the steps taken in an embodiment ofthe invention with respect to the handling of a logically significantwrite request;

FIG. 13 is a flow diagram presenting the steps taken in an embodiment ofthe invention with respect to the handling of a logically insignificantrequest to move a block; and

FIG. 14 is a flow diagram presenting the steps taken in an embodiment ofthe invention with respect to the handling of a request to read a blockin the virtual volume corresponding to the snapshot of the originalvolume.

DETAILED DESCRIPTION OF THE INVENTION

A differential snapshotter does not have to perform any copy-on-writeoperations on disk space that was logically unused at the time of thesnapshot. This is true because the disk blocks that are free on thatsnapshot will never need to be read when the snapshotter produces alogical volume file or directory. For this reason, a differentialsnapshotter may have a bitmap of the blocks on the volume. It may setthe bit to one bit value, such as 1, for blocks that are free at thetime that the snapshot was taken, and it may set to the same value thebits corresponding to blocks that have already had a copy-on-write sincethe time of the snapshot. Clearly, only bits that have the other bitvalue (0 if the first bit value is 1) need to have their blockscopied-on-write. (In the accompanying drawings it is assumed that thefirst bit value, which may be called an “ignore” value, is 1 and thatthe second bit value, which may be called a “protect” value, is 0.However, the invention is of course equally applicable to embodimentswhich use 0 as the “ignore” value and 1 as the “protect” value.)

A defragmentation operation consists of moving some blocks in a file toa location that is free on the volume. The old location of the blockbecomes free and the new location of the block becomes occupied.Therefore, it suffices for a differential snapshotter in accordance withthe invention to be informed that a block is moving from A to B so thatit can change its view of what is free space and what is occupied spacewithout performing any copy-on-write operations but instead simplyupdating a translation table.

FIGS. 1-3 illustrate details of an embodiment of the invention inhandling a block move from block A to block B. Turning to FIG. 1, theprocedure begins at step 11. The differential snapshotter is informedthat a block is moving from A to B by way of a BLOCK_COPY command passeddown by the file system (step 13), rather than a READ_BLOCK followed bya WRITE_BLOCK. This tells the differential snapshotter what operation istaking place. The differential volume snapshotter keeps a bitmap of onebit for every block, where the bit being set indicates that thesnapshotter does not need to take any action when it is written. A clearbit indicates that the snapshotter has to take the copy-on-write. Thesnapshotter keeps a translation table of (Block #→Device, Block #) tosupport reading the snapshot.

If the B bit is clear (step 15), then the snapshotter will copy-on-writethe B block (step 17) before it is written by the move operation (step19) so that there is an entry in the table for the B block (step 21) andthe B bit is set in the bitmap (step 23).

If the B bit is set in the bitmap, there may or may not be an entry inthe table for the B block. If B is free space at the time of thesnapshot then there is no entry in the table. If the A bit is set (step27), then the move operation writes B (step 29) and the snapshotter isdone (step 25). There is no point in doing anything if changes to A canbe ignored.

At this point we have reduced this problem to the case where the bit forblock B is set and the bit for block A is clear. Now we let the movehappen (step 29) and then change the bits to the A bit being set (step31) and the B bit being clear (step 33). We add two entries to thetranslation table: (A→SameDevice, B) (step 35) and (B−>>>A) (step 37)where the−>>>symbol is used to denote that B originally comes from A.The second type of entry provides for fast lookup and, in an embodimentof the invention, it may be used within the same table data structure asthe first type of entry with no extra overhead. Those of skill in theart will recognize that the two kinds of table entry may equivalently bekept in two tables, and that reverse lookup may equivalently beperformed in a translation table using only the first type of tableentry.

FIG. 2 continues the illustration of FIG. 1 where, originally, the bitfor block B was set and the bit for block A was clear, presenting thesteps taken by the snapshotter with respect to a subsequent write ofblock A in FIG. 2A and a subsequent write of block B in FIG. 2B. In FIG.2A, following the completion of the steps illustrated in FIG. 1 (step41), henceforth A can be written freely (steps 43, 45), as its bit isset. In FIG. 2B, following the completion of the steps illustrated inFIG. 1 (step 51), a command to write to B in step 53 will result in acopy-on-write of B (step 55) followed by the write (step 57). Thecopy-on-write of B will then be added to the table in place of theprevious entry (A→SameDevice, B), yielding (A→DiffArea Volume, DiffAreaVolumeOffset) (step 59), the deletion of the (B−>>>A) entry (step 61),and the setting of the B bit (step 63). DiffArea Volume and DiffAreaVolumeOffset represent the differential storage space volume device andblock number, respectively, to which block B is copied.

FIG. 3 continues the illustration of FIG. 1 where, originally, the bitfor block B was set and the bit for block A was clear, the stepsassociated with the move from A to B have occurred (through step 37 ofFIG. 1), and a subsequent write of B has not yet occurred (step 69). Instep 71, a move of block B to block C is initiated. The rules presentedin FIG. 1 then apply, with block B now the old location (correspondingto block A in FIG. 1) and block C the new location (corresponding toblock B in FIG. 1). The B bit is clear (from step 33 in FIG. 1). If theC bit is clear (step 73), then the snapshotter will copy-on-write the Cblock (step 75) before it is written by the move operation (step 77) sothat there is an entry in the table for the C block (step 79) and the Cbit is set in the bitmap (step 81).

If, prior to the move, the C bit is set, we let the move happen (step85) and then change the bits to the B bit being set (step 87) and the Cbit being clear (step 89). However, in preparing to insert(B→SameDevice, C) to the translation table, we find the (B−>>>A) tableentry in place. At this point, the snapshotter effects a composition,yielding the entries (A→SameDevice, C) (step 91) and (C−>>>A) (step 93),which would replace (A→B) and (B−>>>A) (steps 95, 97).

FIGS. 4-14 illustrate aspects of embodiments of the invention in furtherdetail. FIG. 4 illustrates one exemplary computing environment 100within which the present invention may be performed. The environment 100includes a general-purpose stored-program computer machine 110, whichmay be connected to one or more other computer-based resources, such asa remote computer 180 connected to the computer device 110 by a localarea network 171 or wide area network 173. The computer machine 110includes at least one central processing unit 120 connected by a systembus 121 to a primary memory 130. One or more levels of a cache 122,connected to or situated within the processing unit 120, act as a bufferfor the primary memory 130. Programs, comprising sets of instructionsfor the machine 110, are stored in the memory 130, from which they canbe retrieved and executed by the processing unit 120. In the course ofexecuting program instructions, the processing unit 120 retrieves data137 stored in the memory 130 when necessary. Among the programs andprogram modules stored in the memory 130 are those that comprise anoperating system 134.

The exemplary computer machine 110 further includes various input/outputdevices and media for writing to and reading from the memory 130,including secondary storage devices such as a non-removable magnetichard disk 141, a removable magnetic disk 152, and a removable opticaldisk 156. Such computer-readable media provide nonvolatile storage ofcomputer-executable instructions and data; the hard disk 141 is alsocommonly used along with the primary memory 130 in providing virtualmemory. It will be appreciated by those skilled in the art that othertypes of computer-readable media that can provide volatile andnonvolatile storage of data accessible by a computer may also be used inthe exemplary computer environment 100. The computer 110 has a filesystem 142 associated with the operating system 134. The file system 142serves as an interface that maps a set of logically-organized namedfiles to data physically stored on secondary media, such as data storedin clusters or sectors on the hard disk 141.

The diagram of FIG. 5 illustrates an exemplary multi-level secondarystorage system associated with a computer such as the computer depictedin FIG. 4, in the context of which an embodiment of the invention may bepracticed. A differential snapshotter 211 may be regarded as a driverthat mediates between the file system 207 and a block driver 215. Theblock driver 215 provides sector-level access to data contained involumes 221, 225 corresponding to hard disks 219, 223. The snapshotter211 accesses data at the sector level through the block driver 215.Executing programs 201, 205, such as a disk defragmentation utility 203,access stored data at a higher, logical level through the file systeminterface 207.

The differential snapshotter 211 is directed to take a snapshot 217 ofan original disk volume 221 at a specified point in time. The snapshotis a virtual volume 217 containing the versions of files in the volume221 as they existed at the time of the snapshot. Initially, no copyingof data in the original volume 221 is done by the differentialsnapshotter 211. After the time of the snapshot, the snapshotter 211monitors and intercepts efforts by the file system 207 to access datablocks in the original volume 221 on behalf of executing programs 201,203, 205. If the file system 207 attempts to write new data to a block,the snapshotter 211 first consults a bitmap 209 to determine whether itmust preserve the data in that block with a copy-on-write operationbefore the write attempt can proceed. If a copy-on-write is necessary,the snapshotter 211 writes the copy to a special differential storagearea 227, possibly stored in another volume 225 on another disk 223,recording information identifying the copied block and the location inwhich it was copied in one or more table data structures 213.

In embodiments of the invention, the file system 207 has the capacity topass a BLOCK_COPY command to lower-level drivers, enabling lower-leveldrivers to take advantage of hardware acceleration for data blockcopies. In particular, the file system can pass the BLOCK_COPY commanddown to the snapshot driver 211 to request a logically insignificantrelocation of a block from one block location to another in the volume221. Having received the BLOCK_COPY request, which signifies that therequested data movement is not logically significant, the snapshotter211 may be able to avoid performing a copy-on-write by using the bitmap209 and tables 213 in a manner described in further detail below.

The snapshotter 211 also enables the file system 207 to read snapshotversions of files. To the file system 207 the snapshot virtual volume217 appears to be another block device, which the file system 207 canmount. If a requested file that was in the original volume at the timeof the snapshot has been logically changed or nonlogically moved sincethe time of the snapshot, the snapshotter 211, consulting its tables213, will redirect the read request to the appropriate location in thedifferential storage space 227 or in the original volume 221 where thatsnapshot version is stored.

As mentioned above, a bitmap 209 is used by the snapshotter 211 todetermine whether a particular block location must be protected by acopy-on-write operation. In the bitmap 209, a particular bit representsa particular block in the volume 221. When the snapshot is captured, asubset of the blocks in the volume 221 will be logically occupied, inthe sense that they are at that moment being used to implement existingfiles. Another subset of blocks will constitute logically free space. Inthe initial configuration of the bitmap 209, all occupied-space blockswill have their corresponding bits set to “protect,” and all free-spaceblocks will have their bits set to “ignore,” because there is no reasonto perform a copy-on-write for a block that was logically insignificantat the time of the snapshot. In the embodiment illustrated in theexamples of FIGS. 1-3 above and in the examples discussed below, the“ignore” value is 1 and the “protect” value is 0. It should be notedthat once a copy-on-write is performed for a particular block, it is nolonger necessary for the snapshotter 211 to protect that block.

Referring now to FIG. 6, the depicted example illustrates how thesnapshotter handles the straightforward case of a logically significantrequest to write a block location. In FIG. 6A, the snapshotter hasintercepted a WRITE_BLOCK call 301 from the file system, which seeks towrite data 303 at the block location here designated C03 307. The bit317 in the bitmap 319 corresponding to this block is 0, so the block 307must be protected with a copy-on-write operation 311 copying its data todifferential storage space 313 located on a volume 315. FIG. 6B presentsthe view after the copy-on-write has taken place and after the write ofblock C03 323 has been permitted to go forward. The bit 343corresponding to this block 323 is set to 1, since no further protectionof the snapshot version of this block will be needed. The copy-on-writehas been made at location D01 341 in the differential storage space 331.A table data structure 333, mapping blocks 327 to the location 329 atwhich the snapshot versions of those blocks are stored, records the factthat block C03 335 has been copied to differential location D01 337.

Referring now to FIG. 7, the depicted example shows the simplest caseinvolving a logically insignificant block move, such as that which mightbe requested by the file system during the execution of a diskdefragmentation operation following the time of the snapshot. Theexample illustrates how a copy-on-write operation is avoided in such asituation without any loss of information regarding the contents andlocation of the snapshot version of the protected block. FIG. 7Arepresents the situation after the request is intercepted but before itis permitted to proceed. The snapshotter is made aware of the nonlogicalnature of the requested operation by the file system's use of aBLOCK_COPY call 405, in accordance with the invention, instead ofREAD_BLOCK and WRITE_BLOCK calls. Here the request involves therelocation of the data in block C03 407 to block C08 409 in the samevolume 401. In the bitmap 403, the bit 413 corresponding to block C03407 is 0, so some effort must be made to preserve the data in this block407 as the snapshot version of block C03 407. The bit 415 correspondingto the destination block 409 is set to 1, as might be expected if therequested move is a defragmentation operation selecting a currentfree-space location in the volume 401 as the new location for the blockdata being moved. If the bitmap bits 413, 415 corresponding to blocksC03 407 and C08 409 in FIG. 4A had been other than 0 and 1,respectively, the snapshotter would have handled the BLOCK_COPY request405 differently. This will be explained below in the discussion of theflow diagram of FIG. 13.

As a consequence of the requested block move, a logically occupiedblock, which is one of the blocks that must be protected by thesnapshotter, becomes free space, and a free-space block becomes occupiedspace. This change can be reflected in the bitmap simply by exchangingthe bit values 411 in the two bits 413, 415 corresponding to the twoblocks 407, 409 involved in the move. FIG. 7B depicts the situationafter the block move has taken place. Block C08 425 now holds the datathat was previously held in block C03 421, and the corresponding bits423, 427 in the bitmap 419 have been switched. The relocation of thesnapshot version of block C03 435 to block C08 437 is recorded in thetable 429. The mapping here is a translation to another offset in thevolume 417. If the snapshotter receives a request to read the snapshotversion of block C03, it will look up C03 435 in the table 429 and findthat the snapshot copy is currently located at C08 437. The read requestwill be directed to block C08 425.

Referring now to FIG. 8, the depicted example proceeds from the state ofFIG. 7B. In FIG. 8A, two logically significant WRITE_BLOCK requests 551,553 are received for the respective block locations C03 507 and C08 509,the same locations that were involved in the preceding logicallyinsignificant move. The request 551 to write block C03 507 will beallowed by the snapshotter without further action, since itscorresponding bit 513 in the bitmap 503 is set to 1, indicating that itcan be written freely. The bit 515 corresponding to block C08 509,however, is 0, so it must be protected with a copy-on-write before itcan be written. FIG. 5B illustrates the situation following the writes.Blocks C03 521 and C08 525 now hold the new data. The bitmap bit 523corresponding to block C03 521 remains 1, of course. The bit 527corresponding to block C08 525 is set to 1 following the copy-on-write543 depicted in FIG. 8A. The copy-on-write 543 copied the old value ofC08 509, which is the snapshot version of current block C03 521, inlocation D02 547 in the differential storage space 549. In thestorage/translation table 529, the mapping 537 for block C03 535 isupdated accordingly, recording D02 541 as the current location of thesnapshot block C03 539.

Although the diagrams of FIGS. 6-10 show a single mapping table forillustrative simplicity, an additional reverse mapping table may beused. This reverse mapping table may be stored as part of the same datastructure as the direct-mapping translation table, as in the flowdiagrams of FIGS. 1-3, or, in the alternative, it may be maintained as aseparate data structure. A reverse mapping table entry provides, forfast lookup, the mapping from a first block in the original volume to asecond block in the same volume, the second block signifying thelocation whose snapshot version the first block is holding. In theexample of FIG. 8, the snapshotter looks up C08 in the reverse mappingtable, finding C08 mapped to C03, the block location of C08's data atthe time of the snapshot.

While the case of FIGS. 7 and 8 is one in which there was ultimately nonet benefit in the original avoidance of a copy-on-write, in general itis impossible to predict whether there will be a logically significantwrite to a block that has previously been the subject of a logicallyinsignificant move. In the case of a block move pursuant to adefragmentation operation, it is particularly likely that the benefit ofavoiding the copy-on-write will be preserved, since the defragmentationof an entire volume of blocks will involve many moves, only a smallnumber of which can be expected to be the subject of subsequent logicalwrites.

Referring now to FIG. 9, the example depicted therein proceeds from thestate of FIG. 7B and illustrates how the snapshotter handles the move ofa previously-moved block. In FIG. 9A, the snapshotter intercepts a filesystem BLOCK_COPY command 605 for a logically insignificant move 643from block C08 609 to block C10 607, in accordance with the invention.The bitmap bits 615, 613 for these blocks are 0 and 1 respectively, asin the example of FIG. 7, and again the bits 615, 613 will be exchanged645 in order to update the bitmap 603 to reflect the changed blockconfiguration. The snapshotter looks up C08 637 in the reverse mappingtable corresponding to the depicted table 629, finding the reversemapping to C03 635, signifying that block C08 609 is the currentlocation of the snapshot version of block C03 635. As shown in FIG. 9B,representing the state after the data previously stored in block C08 625has been moved to C10 653, the table 655 is updated so that C03 647 ismapped compositionally to C10 649 rather than to C08 641. The bits 627,651 corresponding to blocks C08 625 and C10 653 respectively have beenexchanged, with C10's bit 651 now having the protect value 0.

Referring now to FIG. 10, the example of FIG. 9 is continued in FIG.10A, with a file system attempt 715 to nonlogically move the data inblock C10 709 to block C03 705, using the BLOCK_COPY command 713 inaccordance with the invention. The move destination 705 is also thesnapshot-time location of data currently stored in C10 709. The bitmapbits 711, 707 corresponding to blocks C10 709 and C03 705 are 0 and 1respectively, and the bits are exchanged 717, as seen in FIG. 10Bfollowing the move, where C10's bit 743 is now 1 and C03's bit 741 is 0,as in the original bitmap 703. A lookup of C10 731 in the reversemapping table corresponding to the depicted table 719 reveals C10 731 tobe the current location of the snapshot version of block C03 727. Theappropriate update to the table 745 is the entry 761, 755 mapping C03 toC03, but this is a cycle that may simply be removed from the table.Thus, with respect to block C03 735, the snapshot-time status quo hasbeen restored.

The algorithms applied in the previous examples are presented in furtherdetail in the flow diagrams of FIGS. 11-14. FIG. 11 represents aprocedural overview of an embodiment of the invention. At step 800 theprocedure is begun. In step 802 the snapshotter captures a snapshot ofan original disk volume at a point in time, following which, in step804, it creates the associated bitmap, initially assigning 1 (the“ignore” value) to logically free blocks and 0 (the “protect” value) tologically occupied blocks. In step 806 the snapshotter assumes the roleof monitoring file system requests to access blocks in the originalvolume, as well as the role of enabling the file system to read thesnapshot virtual volume. The method relating to the snapshot of step 802terminates in step 808.

FIGS. 12-14 expand upon the post-snapshot step 806 of FIG. 11. Thesediagrams, like the flow diagrams of FIGS. 1-3, assume that thesnapshotter maintains one translation table holding up to two mappingsfor each original volume block entry a. One mapping, denoted a→b,signifies that block b currently stores the snapshot copy of a. A secondmapping, denoted a−>>>c, the reverse mapping referred to above,signifies that block a currently stores the snapshot copy of c.

The flow diagram of FIG. 12 presents the steps associated with theinterception of a logically significant WRITE_BLOCK from the filesystem. Following the entry into the procedure (step 900), in step 902the snapshotter detects an effort by the file system to logically writeblock k in the original volume. In step 904, the snapshotter checks thevalue of the corresponding bit in the bitmap. If this bit is 1, the filesystem write can proceed (step 914) and the snapshotter exits theprocedure (step 916). If the bit is 0, the block data must be protected.A copy-on-write operation copies the block to a differential storagelocation d (step 906), and the bit corresponding to the copied block isset to 1 (step 908), permitting subsequent accesses of the block to beignored.

In step 910 the snapshotter determines whether there is an entry k−>>>jin the table, reverse-mapping k to some block j in the original volume.If so, block k is the current location of the snapshot version of blockj. The snapshotter removes this reverse mapping (step 918) and thecorresponding direct mapping j→k from the table (step 920). It makes anew table entry j→d, recording differential storage location d as thecurrent location of the snapshot version of j (step 922). At step 914the file system is permitted to write block k, and the snapshotter thenexits (step 916). If, however, there was no reverse-mapping entry for kin the table, the snapshotter makes an entry k→d in the table (step912). Block k can then be written by the file system (step 914), and thealgorithm terminates (step 916).

The flow diagram of FIG. 13 presents the steps associated with theinterception of a file system attempt to nonlogically move a block ofdata from one block location j to another block location k in thevolume. The snapshotter enters the procedure (step 1000) and receivesthe move request (step 1002). The bitmap bits for the source anddestination blocks are examined respectively in steps 1004 and 1006. Ifthe bit corresponding to block j is 1, or if the bit corresponding toblock k is 0, the snapshotter will treat the request as a READ_BLOCK onj to be followed by a WRITE_BLOCK on k using the data stored in j (step1007). To handle the WRITE_BLOCK on k, the snapshotter follows theprocedure outlined in FIG. 12 (step 1009).

If the bit corresponding to j is 0 and the bit corresponding to k is 1,the optimization associated with the invention can be realized. Thesnapshotter determines whether there is a reverse-mapping entry j−>>>iin the table mapping j to some block i in the same volume (step 1008).If so, j is currently storing the snapshot version of block i. Thedirect-mapping table entry i→j is deleted (step 1010), and thecorresponding reverse-mapping table entry j−>>>i is deleted (step 1012).If i and k are not the same block location, determined at step 1014, adirect-mapping entry i→k is added to the table (step 1016), as is thecorresponding reverse mapping k−>>>i (step 1018). These two steps areskipped if i and k are the same. In either case, the bits correspondingto j and k are swapped (step 1024), the block move is allowed to proceed(step 1040), and the procedure terminates (step 1042), the block movehaving been achieved without a copy-on-write operation.

Finally, the flow diagram of FIG. 14 presents the steps taken by thesnapshotter in enabling the file system to read the virtual snapshotvolume. The procedure begins at step 1100, and at step 1102 a filesystem request to read a particular block v in the snapshot volume isreceived. The snapshotter determines whether there is an entry v→w inthe table (step 1104). If such an entry exists, it signifies that thesnapshot copy of block v is stored at another location w, either in thesame volume or in the differential storage space. The snapshotterdirects the file system read to w (step 1106), and the procedureterminates (step 1110). If there is no entry for v in the table, thesnapshot copy of block v is the same as the current contents of block vin the original volume. The snapshotter therefore directs the read tothe actual block v (step 1108), and the procedure terminates (step1110).

The foregoing detailed description discloses a method for capturing andmaintaining a differential snapshot in which logically significantwrites of data blocks are distinguished from logically insignificantmoves of block data. The ability of the snapshotter to make thisdistinction is accomplished by an innovation in the file system wherebya BLOCK_COPY command can be passed to drivers below the file systemlevel, which also enables those drivers to take advantage of hardwareacceleration of data block copies. With respect to the differentialsnapshotter, substantial economies of processing time and storage spaceare achieved. While, as those skilled in the art will readily recognize,the invention is susceptible to various modifications and alternativeconstructions, certain illustrative embodiments have been shown in theaccompanying drawings and have been described above in detail. It shouldbe understood, however, that there is no intention to limit theinvention to the specific forms disclosed. On the contrary, theintention is to cover all modifications, alternative constructions, andequivalents falling within the spirit and scope of the invention.

1. In a data storage system comprising a file system and one or more disk volumes, each volume comprising files and blocks, wherein each file is implemented by a set of one or more blocks, wherein blocks currently implementing a file are occupied-space blocks and all other blocks are free-space blocks, a method comprising: on a computing device including a processor taking a snapshot of an original volume using a first differential snapshotter program that is configured to mediate between the file system and a block driver; wherein taking the snapshot includes creating a bitmap comprising a plurality of bit spaces, wherein each bit space corresponds to a respective block in the original volume, wherein each bit space is set either to an ignore value or to a protect value, and wherein initially a bit space is set to the ignore value when the respective block that corresponds to the bit space is a free-space block when the snapshot is taken, and a bit space is set to the protect value when the respective block that corresponds to the bit space is an occupied-space block when the snapshot is taken; wherein at a time of the snapshot of the original volume data remains occupied from the original volume to the snapshot of the original volume; the differential snapshotter program performing actions comprising: intercepting a move request by the file system to move data blocks in the original volume; preventing the move to occur until a determination is made as to the data in the data blocks is to be preserved; monitoring moves of occupied-space blocks in the original volume, wherein moving an occupied-space block comprises transferring contents from within the occupied-space block to a second block, which is one of the free-space blocks, the second block becoming an occupied-space block, and the first block becoming a free-space block; the differential snapshotter program performing actions comprising: intercepting a write request by the file system to write data blocks in the original volume; preventing the write to occur until a determination is made as to the data in the data blocks is to be preserved; monitoring writes of blocks in the original volume; wherein monitoring the writes of the blocks includes determining which blocks to protect by copy-on-write operations before the writing of the blocks such that data in that block is preserved and which blocks not to protect by the copy-on-write operations; wherein monitoring the moves and the writes comprises using the first program to intercept efforts by the file system to access blocks in the original volume; and producing a snapshot version of blocks in response to a read request using the information on which blocks to protect by the copy-on-write operations and which blocks not to protect.
 2. The method of claim 1, wherein monitoring the moves includes intercepting a block copy command passed down by the file system.
 3. The method of claim 1 wherein moves of blocks include moves that are associated with a defragmentation operation.
 4. The method of claim 1, wherein monitoring the moves, monitoring the writes and producing the snapshot version of blocks includes recording information regarding a current actual location for the snapshot version of the blocks in the original volume.
 5. The method of claim 4, wherein monitoring a write of a block further comprises: when the bit space in the bitmap corresponding to the block is set to the protect value, copying the block to a location in a differential storage space; setting the bit space to the ignore value; when the block to be written is recorded as the current actual location of one of the snapshot versions that relates to a different block in the original volume, recording the location of the copy in the differential storage space as the current actual location for the snapshot version of that different block, and otherwise recording the location of the copy in the differential storage space as the current actual location for the snapshot version of the block to be written; and permitting the write to proceed; and when the bit space in the bitmap corresponding to the block is set to the ignore value, permitting the write to proceed.
 6. The method of claim 5, further comprising, when the bit space in the bitmap corresponding to the block to be written is set to the protect value: deleting each entry in a translation table that indicates that the block to be written is the current actual location for the snapshot version of a different block, and recording in a new table entry the location of the copy in the differential storage space as the current actual location for the snapshot version of the block to be written.
 7. The method of claim 4, wherein monitoring a move of a first block to a second block comprises: when the bit space in the bitmap corresponding to the first block is set to the protect value and the bit space in the bitmap corresponding to the second block is set to the ignore value then: permitting the move to proceed; when the first block is recorded as the current actual location of the snapshot version of a third block in the original volume: recording the second block as a new current actual location of the snapshot version of the third block, otherwise recording the second block as the current actual location of the snapshot version of the first block and exchanging the bits in the bit spaces in the bitmap corresponding to the first block and the second block, so that the bit space corresponding to the first block is set to the ignore value, and the bit space corresponding to the second block is set to the protect value; when the bit space corresponding to the second block is set to the protect value: copying the second block to a location in a differential storage space; setting the bit space corresponding to the second block to the ignore value; when the second block is recorded as the current actual location of the snapshot version of a different block in the original volume: recording the location of the copy in the differential storage space as the current actual location of the snapshot version of that different block; otherwise recording the location of the copy in the differential storage space as the current actual location of the snapshot version of the second block; and Q permitting the move to proceed; when the bit space corresponding to the first block is set to the ignore value, permitting the move to proceed.
 8. The method of claim 7, wherein a translation table is used to record information regarding the current actual location for the snapshot version of the blocks, the method further comprising, when the bit space corresponding to the first block is set to the protect value, if the bit space corresponding to the second block is set to the ignore value, and when the first block is recorded in the table as the current actual location of the snapshot version of a third block in the original volume: when the second block and the third block are the same block, deleting each entry in the table that indicates that the first block is the current actual location of the snapshot version of the second block, and when the second block and the third block are not the same block, recording in a new table entry the second block as the new current actual location of the snapshot version of the third block.
 9. The method of claim 4, wherein producing the snapshot version of blocks in response to the read request comprises directing the read request to the current actual location of the snapshot version of the blocks.
 10. The method of claim 9, wherein a translation table is used to record information regarding the current actual location of the snapshot version of the blocks, the method further comprising: when the block to be read has a table entry indicating the current actual location of the snapshot version, redirecting the read to that location, and otherwise, directing the read to the version of the block in the original volume. 